The present application generally relates to exhaust aftertreatment systems, and more particularly, but not exclusively, to selective catalytic reduction (“SCR”) systems. Presently available SCR systems adsorb ammonia (NH3) on a catalyst and then react the NH3 with NOx to reduce the NOx emissions. The NH3 is typically stored as a less reactive composition, e.g. urea, and hydrolyzed into NH3 in the exhaust system as required to reduce the NOx emitted by the engine. Immediately after an engine startup event, under certain conditions the exhaust system is not warm enough to sufficiently hydrolyze urea. A lag time in the ability to deliver NH3 to the reduction catalyst can increase emissions of the system. If the NOx reaches the reduction catalyst and no NH3 is available, a higher portion of the NOx will slip out of the system as increased emissions.